A power conversion control device has been known as an example of a device that drives an alternating current motor, such as a synchronous machine, a reluctance machine, an induction machine, or an induction-synchronous machine, without a position sensor. The alternating current motor has been known as disclosed in, for example, Japanese Laid-open Patent Publication No. 2007-300780. The power conversion control device includes a power converter, a current detector, a frequency calculation unit that determines an operating frequency of the power converter, an integration calculation unit that calculates and outputs a phase angle signal from an output of the frequency calculation unit through an integral operation, an orthogonal two-axis conversion unit, a two-axis current control unit, a PWM signal generating unit, and a PWM controller that controls the power converter in accordance with a PWM signal.
The orthogonal two-axis conversion unit calculates and outputs a two-axis current that has an active component and a reactive component by performing orthogonal two-axis conversion on the basis of the detection signal of the current detector and the phase angle signal of the integration calculation unit. A two-axis current setting unit outputs a current command value of the active component and a current command value of the reactive component of the two-axis current. The two-axis current control unit calculates an amount of an error from a difference between the output of the orthogonal two-axis conversion unit and the output of the two-axis current setting unit, and outputs a voltage magnitude command value according to the amount of an error for each two-axis component. The PWM signal generating unit generates a PWM signal for controlling the power converter on the basis of the output of the two-axis current control unit and the phase angle signal of the integration calculation unit. Then, in order that the position sensor may not be indispensable, the frequency calculation unit determines the operating frequency of the power converter to guide a reactive component-axis voltage magnitude command value to zero among the voltage magnitude command values that are the output of the two-axis current control unit.
The conventional power conversion control device realizes a high-efficiency operation by a simple command operation without using a position and speed sensor by employing such a configuration.
The conventional power conversion control device has few control parameters, which are required for high-efficiency control. However, for example, the active component-axis voltage magnitude command value that is the output of the two-axis current control unit is a value that is obtained by multiplying it by a gain value KG that is an inverse number of an induced voltage constant obtained from the reactive component-axis voltage magnitude command value. Moreover, it is preferable that the gain value KG is determined as the operating frequency of the power converter. Japanese Laid-open Patent Publication No. 2007-300780 further discloses that a speed estimation method is performed in consideration of the size of a voltage drop caused by wirewound resistance and inductance. Herein, an electric motor constant is directly used or an electric motor constant is simply replaced by a gain value Kγ.
Furthermore, it is assumed that the two-axis current setting unit controls an electric motor in which an induced voltage is basically larger than the size of a voltage drop caused by wirewound resistance and inductance and thus an angle (phase difference angle β) formed by a voltage vector and the q axis perpendicular to a magnetic flux component axis (d axis) becomes small, that is to say, an electric motor in which a command value can be determined and a reactive power can be ignored in such a manner that a power factor is “1” basically. The setting method of the γ-axis current Iγ directly using an electric motor constant and a power factor angle β is disclosed. In the setting method, it is required to vary a control gain in accordance with an operating state, or a computation method of a current command becomes complicated.
In this way, Japanese Laid-open Patent Publication No. 2007-300780 assumes that a control target is an electric motor that basically uses a permanent magnet. Therefore, when a control target is an electric motor that is not easily operated like a synchronous reluctance motor when the power factor is “1” or an electric motor in which the variation of a motor characteristic is large, it is hard to say that two operations of a position sensorless operation and a simple high-efficiency operation can be compatible.